Packaged microelectronic assemblies, such as memory chips and microprocessor chips, typically include a microfeature die mounted to a substrate and encased in a plastic protective covering. The die includes functional features, such as memory cells, processor circuits, and interconnecting circuitry. The die also typically includes bond pads electrically coupled to the functional features. The bond pads are electrically connected to pins or other types of terminals that extend outside the protective covering for connecting the die to busses, circuits, and/or other microelectronic assemblies.
In one conventional arrangement, microelectronic dies are attached to other microelectronic dies, interposer boards, or other support substrates using solder balls. Small balls of solder are attached to bond pads located at the surface of the die using a reflow process. The die, along with the attached solder balls, is then positioned over the support substrate so that the solder balls are aligned with corresponding bond pads at the surface of the support substrate. The support substrate can include a no-flow underfill material containing a flux to facilitate the electrical connection between the bond pads and the solder balls, and an epoxy to facilitate the physical connection between the support substrate and the die. The die and support substrate are then brought toward each other such that the solder balls carried by the die contact the bond pads carried by the support substrate. A subsequent reflow process is then used to fuse the solder balls to the support substrate bond pads.
One potential drawback with the foregoing approach is that not all the solder balls may make contact with the corresponding bond pads at the support substrate. For example, some of the solder balls may be misshapen or smaller than normal, and accordingly a gap may exist between these solder balls and the corresponding bond pads. During the reflow process, this gap may not seal, and the result may be an open circuit between the die bond pad and the corresponding support substrate bond pad.
Another potential drawback with the foregoing approach is that it may result in a vulnerable mechanical connection between the die and the support substrate. For example, when the die with pre-attached solder balls is brought into contact with the support substrate, air bubbles may become trapped between the lower surface of the die and the upper surface of the underfill material. During subsequent high temperature processes, the air trapped in this region may expand and force the die away from the support substrate, damaging the mechanical and/or electrical connections between these components.
In another conventional arrangement, flux is applied to the support substrate bond pads, in the absence of a no-flow underfill material, to facilitate the electrical connection between the bond pads carried by the support substrate, and the solder balls carried by the die. After the solder balls have been reflowed to establish this connection, a capillary underfill material is applied to fill in the interstices between the die and the support substrate. One drawback with this approach is that it requires the application of flux to the substrate bond pads, which increases processing time. Another drawback is that residual flux can affect the ability of the capillary underfill material to adhere to the die and the substrate. Accordingly, the substrate must be cleaned prior to applying the underfill material, which further increases the processing time.
In light of the foregoing potential drawbacks, existing processes may create at least some faulty packaged dies. In order to increase the efficiency and overall throughput of the manufacturing process for such dies, it is desirable to increase the robustness of both the mechanical and electrical connections between microfeature dies and the structures to which they are attached.